This invention relates to the contacting of solids with gases and is particularly concerned with an apparatus in which particulate solids, such as particles of green needle coke, are passed downwardly on a declined bed support while in contact with a gas, preferably a hot gas.
U.S. Pat. Nos. 4,521,278 and 4,545,859, the disclosures of which are hereby incorporated by reference in their entireties, both disclose processes for producing a high quality calcined needle coke from highly friable green needle coke. In the process disclosed by U.S. Pat. No. 4,521,278, green needle coke is heated at a temperature between 875.degree. F. and 1200.degree. F. for a time sufficient to reduce its friability, and then, without cooling below 500.degree. F., the heated green needle coke is calcined at a temperature above 2000.degree. F. In the process disclosed by U.S. Pat. No. 4,545,859, the green needle coke is heated at a temperature between about 935.degree. F. and about 1100.degree. F. for a time sufficient to reduce its friability, and then, after being cooled to a temperature below 250.degree. F., the needle coke is calcined at a temperature above 2000.degree. F. In both processes the lower temperature heating or precalcination step, as opposed to the higher temperature calcination step, is carried out in a declined bed-type heater.
In the declined bed-type heaters described in U.S. Pat. Nos. 4,521,278 and 4,545,859, the friable green needle coke is preheated prior to calcination by passing the coke down a bed support declined from the horizontal at an angle sufficiently large that the coke gravitates down the bed support at a relatively uniform bed depth while being contacted with hot gases that pass upward through openings in the bed support. Normally, declined bed heaters are very effective gas-solids heat transfer devices. Major problems with heat transfer, however, have recently been unexpectedly encountered in a commercial size declined bed-type heater. These heat transfer problems were found by the present inventor to be caused, at least in part, by the preferential flow of the hot gases along the sidewalls of the heater where the gas does not contact the coke passing down the bed support. This gas bypassing results in maldistribution of the hot gases as they pass through the bed support openings in contact with the particles of green needle coke and thereby makes it difficult to obtain uniform heat transfer to the individual coke particles. The gas bypassing also results in the need to increase the flow of hot gases to reach a given degree of solids heating.
Accordingly, it is one of the objects of the present invention to provide a declined bed-type heating apparatus that can be commercially used to uniformly heat solids such as needle coke without a decrease in heat transfer efficiency caused by the preferential flow of hot gases along the sidewalls of the heating apparatus. This and other objects of the invention will become more apparent in view of the following description of the invention.